Chlorination is the most widely used technology for wastewater disinfection. Chlorine can be added to treated effluent in several forms, most commonly as chlorine gas, sodium hypochlorite solution, or calcium hypochlorite tablets. For disinfection of effluent from conventional wastewater treatment processes that are not designed to remove nitrogen, chlorine reacts with ammonia nitrogen to form chloramines that are effective disinfectants. Inorganic chloramines consist mainly of monochloramine, and are referred to as “combined chlorine”.
Over the past decade, removal of nitrogen from municipal wastewater has become an ever more common treatment objective. Consequently, an increasing number of conventional wastewater treatment systems have been upgraded to include biological nitrogen removal. Effluent from these plants is typically filly nitrified, i.e., it contains very low levels (<1 mg nitrogen per liter N/L)) of ammonia nitrogen. Free chlorine (hypochlorous acid and hypochlorite ion), instead of combined chlorine, becomes the main disinfectant at work if chlorine is used for disinfection of fully nitrified effluent. Free chlorine residual will be present if the chlorine dose exceeds approximately ten times the ammonia nitrogen concentration (on a weight basis) in the water. This process is referred to as the “breakpoint chlorination” process. Free chlorine species are strong oxidants and react at a much faster rate than combined chlorine. Free chlorine reacts with natural organic matters such as humic and fulvic substances in the effluent to form such disinfection byproducts (DBPs) as trihalomethanes (THMS) and haloacetic acids (HAAs). Because of their potential adverse effect on human health, the U.S. Environmental Protection Agency (USEPA) has set drinking water standards for total THMs (four chlorinated and brominated compounds) and HAA5 (five chlorinated and brominated haloacetic acids) at 80 μg/L (micrograms per liter, or parts per million) and 60 μg/L, respectively. Other DBPs that may be generated from the breakpoint chlorination process include cyanide and cyanogen chloride. Toxicity to aquatic life is another potential concern with breakpoint chlorination.
To minimize the formation of THMs and HAAs, some wastewater treatment plants that produce fully nitrified effluent continue to use chloramination for disinfection. This is accomplished by either adding pre-formed chloramines, or low levels of ammonia nitrogen followed by chlorine to form chloramines. However, it was recently found that chloramines are precursors to nitrosamines, a group of compounds considered to be extremely potent carcinogens. The most studied nitrosamine in wastewater treatment is N-nitrosodimethylamine (NDMA). The USEPA has established a 1 in 1,000,000 cancer risk at 0.7 ng/L (nanograms per liter, or parts per trillion) for NDMA, and the California Department of Health Services has set a drinking water Notification Level for NDMA at 10 ng/L. NDMA is formed when chloramines react with organic nitrogen-containing precursors such as dimethylamine (DMA). DMA is present in filly nitrified effluent, and is a key component in the cationic polymer commonly used to enhance floe settling during wastewater treatment. Dependent on the amount of precursors in water, significant levels (up to thousands ng/L) of NDMA may be formed from chloramination.
The Sanitation Districts of Los Angeles County (Districts), as well as many other such sanitation facilities, operate several water reclamation plants (WRPs) that produce fully nitrified effluent suitable for reuse applications. FIG. 1 depicts the typical treatment processes used to produce tertiary effluent at such WRPs. The processes include primary settling, aeration and secondary settling (the activated sludge process), media filtration, disinfection with chloramination, and dechlorination before discharge or reuse. The activated sludge process is designed and operated to biologically remove nitrogen so that the effluent is fully nitrified. In addition, Mannich type polymer is added before the secondary settling tanks to enhance floe settling.
While there may be multiple pathways to form NDMA, formation has not been determined. A study by Mitch and Sedlak suggested that formation of NDMA by reaction between monochloramine and organic nitrogen species, such as dimethylamine (DMA) via unsymmetrical dimethylhydrazine (UDMH) pathway could explain observed NDMA formation in full-scale treatment plants (Mitch and Sedlak, 2002). The proposed mechanism is described as:NH2Cl+NH2(CH3)2→(CH3)2NCl+NH4+(monochiloramine+DMA→chlorinated-DMA (CDMA)+NH4+)(CH3)2NCl+NH3→NH2N(CH3)2+HCl(CDMA+NH3→UDMH+HCl)NH2N(CH3)2+NH2Cl→(CH3)2N2O+others(UDMH+monochloramine→NDMA+others products)
Because of the use of chloramination for disinfection, NDMA formation has been observed in the Districts' WRPs. Although there is no federal or California drinking water standard for NDMA at present, the levels of NDMA formed during chloramination of wastewater are an important concern for the reuse of municipal wastewater. Therefore, it is desirable to prevent NDMA formation in the existing disinfection process.
One of the alternatives studied was breakpoint chlorination. Breakpoint chlorination was tested at two of the Districts' WRPs. Results from these studies indicated that breakpoint chlorination effectively inactivated total coliform and significantly reduced NDMA formation (Tang et al. 2006). However, breakpoint chlorination occasionally generated levels of total THMs higher than the drinking water standard. It was necessary to find another solution to the problem.